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OPTICAL%20ISOMERISM

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OPTICAL ISOMERISM R W Grime Ripon Grammar School Molecular mirror images Optical isomerism and chirality Enantiomers and racemates Self-Test: recognising optical ... – PowerPoint PPT presentation

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Title: OPTICAL%20ISOMERISM


1
OPTICAL ISOMERISM
R W Grime Ripon Grammar School
Molecular mirror images
Optical isomerism and chirality
Enantiomers and racemates
Self-Test recognising optical isomers from
structural formulae
Polarimetry recognising optical isomers by
experiment
Examples of optical isomers and their importance
thalidomide
carvone
limonene
2
  • All molecules have a mirror image but for most
    molecules it is the same molecule.

3
  • For some molecules the mirror image is a
    different molecule (the mirror image is
    non-superimposable).

4
  • Left and right hands are an example of
    non-superimposable mirror images.

5
  • This usually happens when a molecule contains a C
    atom with four different groups attached (chiral
    / asymmetric C).
  • Such molecules are said to be chiral or optically
    active.

6
  • The optical isomers are called enantiomers.
  • These are distinguished by /-, D/L or more
    correctly R/S.
  • A 50/50 mixture of the two enantiomers is called
    a racemic mixture or a racemate and shows no
    optical activity

7
  • TASK Some of the following molecules are
    optically active. For each one, click its name
    below and decide whether it is optically active
    or not. Click again to see if you are correct.

a) propan-2-ol
e) butanone
b) 2-chlorobutane
f) 2-methylbutanoic acid
c) 1-chlorobutane
g) butan-2-ol
d) 3-methylhexane
h) 1-chloro-3-methylpentane
8
  • propan-2-ol

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
9
  • 2-chlorobutane

Click here to go back to the optical isomerism
task
10
  • 1-chlorobutane

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
11
  • 3-methylhexane

Click here to go back to the optical isomerism
task
12
  • butanone

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
13
  • propan-2-ol

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
14
  • 2-methylbutanoic acid

Click here to go back to the optical isomerism
task
15
  • butan-2-ol

Click here to go back to the optical isomerism
task
16
  • 1-chloro-3-methylpentane

Click here to go back to the optical isomerism
task
17
  • Molecules that are optical isomers are called
    enantiomers.
  • Enantiomers have identical chemical and physical
    properties, except
  • Their effect on plane polarised light
  • Their reaction with other chiral molecules

18
  • Light is a form of electromagnetic radiation.

19
  • The wave vibrations are perpendicular to the
    direction of travel of the wave.

20
  • Optical isomers rotate the plane of plane
    polarised light.

21
  • POLARIMETERS can be used to analyse the effect
    optical isomers have on plane polarised light

Heriot Watt University has a web page with an
interactive tutorial and self-test questions
about this topic http//scholar.hw.ac.uk/site/che
mistry/activity5.asp?outline
22
  • Chiral molecules often react differently with
    other chiral molecules.
  • This is like the idea that a right hand does not
    fit a left handed glove the molecule must be
    the correct shape to fit the molecule it is
    reacting with.
  • Many natural molecules are chiral and most
    natural reactions are affected by optical
    isomerism.

23
  • For example, most amino acids (and so proteins)
    are chiral, along with many other molecules.
  • In nature, only one enantiomer usually occurs
    (e.g. all natural amino acids rotate polarised
    light to the left).

24
  • Many drugs are optically active, with one
    enantiomer only having the beneficial effect.
  • In the case of some drugs, the other enantiomer
    can even be harmful, e.g. thalidomide.

25
  • In the 1960s thalidomide was given to pregnant
    women to reduce the effects of morning sickness.
  • This led to many disabilities in babies and early
    deaths in many cases.

The photographs are both from Molecule of the
Month at Bristol University http//www.chm.bris
.ac.uk/motm/thalidomide/start.html
26
S thalidomide (effective drug) The body
racemises each enantiomer, so even pure S is
dangerous as it converts to R in the body.
R thalidomide (dangerous drug)
27
  • Thalidomide was banned worldwide when the effects
    were discovered.

28
Your nose is chiral too!
29
S carvone (caraway seed)
R carvone (spearmint)
Caraway Seed has a warm, pungent, slightly bitter
flavour with aniseed overtones.
30
S limonene (lemons)
R limonene (oranges)
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